Grafted copolymers of nitrogenous, heterocyclic monomers with polyolefins having pendant ethylenically unsaturated moieties have previously been proposed for use in lubricating oils as viscosity index improving (VII) agents and as dispersants for keeping the insoluble materials in the crankcase of an internal combustion engine in suspension. Among many graftable polyolef ins suggested for this use are ethylene-propylene polyolefins, grafted with 0.3% by weight of N-vinylimidazole. U.S. Pat. No. 4,092,255, col. 10, ll. 52-53. Other examples in the same patent are "statistic" copolymers, which are distinguished from grafted copolymers. U.S. Pat. No. 4,092,255, col. 4, ll. 5-13.
Another material which has been suggested for use as both a viscosity index improver and a dispersant is a polymer containing N-vinyl pyrrolidone and an alkyl methacrylate. U.S. Pat. No. 4,146,489, col. 1, ll. 51-62.
Previous dispersant viscosity index improvers ("DVII's"), including grafted copolymers of N-vinylimidazole and olefinic polymers, have typically provided an ADT (asphaltene dispersancy test) value of from about 2 to about 4. A dispersant viscosity index improver (DVII) having a higher ADT value would be able to disperse the insoluble material in a lubricating oil composition when less of the dispersant is used in the oil. Thus, a DVII having a higher ADT value would be a better dispersant than the currently available materials.
Now consider the manufacture of grafted polyolefins. Grafted polyolefins for use as lubricating oil additives have previously been prepared by dissolving the selected polyolefin in a solvent (which may be a lubricating oil base stock), adding an organic peroxide as a free radical generator (also referred to in this specification as an initiator), holding the mixture at an elevated temperature to form active sites on the polyolefin, adding the graftable monomer, and allowing the mixture to react at an elevated temperature for long enough to form the desired grafted polyolef in. U.S. Pat. No. 4,092,255, col. 4, l. 54, to col. 5, l. 12.
The prior art also suggests that the grafting reaction to form a dispersant VII grafted polyolef in can be controlled to avoid by-products by combining the polyolefin, graftable monomer, and initiator at a temperature below the initiation (reaction) temperature of the initiator, then heating the mixture to above that initiation (reaction) temperature to begin the reaction. U.S. Pat. No. 4,146,489. Example 1 of the '489 patent suggests that the initiation (reaction) temperature of di-t-butyl peroxide is between 160.degree. C. and 170.degree. C. Addition of the initiator in two stages is suggested in Example 4 of the same patent. A grafted polyolefin containing 1-10% by weight, preferably 2-6% by weight, most preferably about 3% by weight of the grafted monomer is taught. '489 patent, col. 3, ll. 11-15.
One problem with prior grafted polyolefins is their limited shear stability--the ability to withstand extensive shearing, as in an internal combustion engine, without losing potency as viscosity-index improving additives. U.S. Pat. No. 4,146,489, col. 5, ll. 48-58, states that: "During the grafting reaction, noticeable thickening takes place, and evaluation of the grafted polyolefin indicates that shear stability deteriorates during the grafting reaction. This very likely results from crosslinking that may occur as part of the reaction. Although it is possible to eliminate this crosslinking, the products so prepared generally are inferior dispersants. Hence, it appears to be inherent to some extent in the grafting process of this invention that to obtain optimum dispersancy, some compromise in shear stability is necessary."
The '489 patent also points out that the shear stability of the grafted copolymer can be improved by mechanical or thermal degradation of the polyolefin to reduce the content of high-molecular-weight species which are readily broken down by such processes. However, it is not desirable to carry out the time-consuming and expensive processes necessary to mechanically or thermally degrade the polyolefin.
Another manner proposed for preventing side reactions in the grafting process, which are said to include cross-linking of polyolefin chains, homopolymerization of the graftable monomer, or functionalization of the grafted polyolefin, is to run the reaction at a relatively high temperature, such as 190.degree. C. or more if, for example, di-t-butyl peroxide is used as an initiator. This expedient is also said to allow the proportion of grafted monomer in the resulting product to be increased, as well. U.S. Pat. No. 4,810,754, col. 2, ll. 19-43.
A grafting reaction has been carried out, according to Example 1 of U.S. Pat. No. 4,810,754, by adding the initiator (di-t-butyl peroxide) and the graftable monomer (2-vinylpyridine--molecular weight 105.14) over a period of 45 minutes to an ethylene-propylene polyolefin reaction mixture maintained in a solvent mineral oil at 190.degree. C. The resulting grafted polyolefin was said to contain as much as 0.17% nitrogen by one analysis. If accurate, this nitrogen level would indicate 1.3% by weight of vinylpyridine monomer grafted on the polyolefin.